1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device in which a pixel driving thin film transistor(TFT) and a driver circuit region are integrated on the same panel.
2. Discussion of the Related Art
An active matrix liquid crystal display used for a liquid crystal display (LCD) with large area and high resolution includes a pixel driving TFTs for driving display pixels and a driver circuit TFTs for applying signals to gate bus lines and data bus lines coupled to the pixel driving TFTs.
In general, there are two types of driver circuit unit. The first one is such that the driver circuit unit is a separate integrated circuit for driving signal lines, and the integrated circuit is attached to one end of the substrate of the liquid crystal panel. The second type is such that the driver circuit unit is formed on an LC panel as one piece. In the second type, typically, complementary metal oxide semiconductor TFTs (CMOS TFTs) using polycrystalline silicon (p-Si) with a high electric field effect mobility have been used for the driver circuit. Since these CMOS TFTs are made of p-Si, the switching speed is much higher than that for amorphous silicon TFTs. Further, because the driver circuit TFTs and the pixel driving TFTs may be fabricated at the same time, the fabrication cost can be reduced.
FIGS. 1A to FIG. 1H are drawings showing a method of fabricating a conventional liquid crystal display device integrated with a driver circuit. A substrate 1 is divided into a pixel region and a driver circuit region. The driver circuit region is further divided into a part B for N-channel metal oxide semiconductor TFTs(NMOS TFTs) and a part A for P-channel metal oxide semiconductor TFTs(PMOS TFTs).
As shown in FIG. 1A, a buffer layer 3 is provided on the substrate 1, and then p-Si is deposited on the buffer layer 3 and patterned to form semiconductor layers 4a, 4b, 4c in the pixel region and the driver circuit region. As a result, one semiconductor layer 4c is provided on the pixel region and two semiconductor layers 4a, 4b are formed on the driver circuit region. The semiconductor layer 4c formed on the pixel region is for a pixel driving TFT for driving the pixel. The semiconductor layers 4b, 4c are for NMOS and PMOS TFTs, respectively.
As shown in FIG. 1B, an insulating layer 5, such as SiO.sub.2 or SiNx, a metal layer 6, such as Al, Al alloy, or Cr, and photoresist 20a are successively formed over the substrate 1. The insulating layer 5 and the metal layer 6 are patterned by photolithography to form a gate insulating layer 5 and gate electrodes 6a, 6b, 6c, as shown in FIG. 1C. Subsequently, as shown in FIG. 1D, n.sup.- doping in low impurity-concentration is executed into the entire surface of the substrate 1 to form an n.sup.- layer 12b and a channel layer 12a in each of the semiconductor layers 4a, 4b, 4c. Here, the n.sup.- type ions are blocked by the gate electrodes 6a, 6b, 6c, and do not reach the channel layer 12a.
Thereafter, as shown in FIG. 1E, in order to form a lightly doped drain (LDD) structure, n.sup.+ doping in high impurity concentration is executed after a photoresist layer 20b is deposited over the substrate and patterned to cover part A of the driver circuit region, the gate electrode 6c, and some part of the semiconductor layer 4c of the pixel region.
Accordingly, n.sup.+ layers 12c are formed by the n.sup.+ doping in the semiconductor layers of the pixel region and part B of the driver circuit region. The LDD structure is made by forming n.sup.+ layer 12c with high impurity-concentration and n.sup.+ layer 12b with low impurity-concentration in the semiconductor layer 4c of the pixel region
As shown in FIG. 1F, after photoresist layer 20b is removed, another photoresist layer 20c is deposited and patterned to cover the pixel region and part B of the driver circuit region. Then, p.sup.+ doping with high impurity-concentration is executed to form p.sup.+ layer 12d in part A of the driver circuit region.
Such a method of forming a p.sup.+ layer is called a counter doping method. When the n.sup.- doping is executed, the concentration of ions doped in the semiconductor layers is about 10.sup.16 -10.sup.18 atoms/cm.sup.3, and when the p.sup.+ doping is executed, the concentration of ions doped in the semiconductor layer is about 10.sup.19 -10.sup.21 atoms/cm.sup.3. Therefore, the n.sup.- layer becomes a p.sup.+ layer by the p.sup.+ doping. This way, TFTs having an LDD structure including the n.sup.+ layers 12c and n.sup.- layers 12b are formed in the pixel region, and CMOS TFTs having NMOS TFTs including the n.sup.+ layers 12c and PMOS TFTs including the p-layers 12d are formed in the driver circuit region.
As shown in FIG. 1G, after the photoresist layer 20c is removed, an insulating layer 7, such as SiNx or SiO.sub.2, is deposited over the entire surface of the substrate 1. Contact holes are then formed in the insulating layer 7, and a metal such as Al is deposited and patterned to form source/drain electrodes 8.
And, as shown in FIG. 1H, after indium tin oxide (ITO) is deposited and patterned to form a transparent electrode 9 a passivation layer 10 is further deposited!. Finally, TFTs having an LDD structure are formed in the pixel region and CMOS TFTs are formed in the driver circuit region, thereby completing a liquid crystal display device integrated with a driver circuit.
However, the liquid crystal display device integrated with a driver circuit formed by the above-mentioned method have complex processes.